The present invention relates to a portable electronic apparatus, an IC card and a reader/writer. More particularly, the invention relates to a non-contact type IC card system which samples results of detection at high speed to decode data based on the signal level distribution of a detected signal and which further decodes data by detecting a correlation value representing the sameness between a clock signal and the detected signal, so that bit errors are sufficiently reduced during deterioration of the detected signal obtained by detection of a high-frequency signal induced on an antenna.
Earlier development, non-contact type IC card systems used in automated ticket inspection systems at railway stations involve having suitable data communicated wirelessly between an IC card and a reader/writer to read data from the card and update the data, if necessary.
FIG. 11 is a block diagram of an IC card system 1. The IC card system 1 in FIG. 11 includes a non-contact type IC card 2, a reader/writer 3 for gaining access to the IC card 2, and a personal computer 4. The personal computer 4 processes results of access by the reader/writer 3 to the IC card 2 in order to manage illustratively the card holder entering or leaving rooms and passing through automated wickets of railway stations.
In the reader/writer 3, a digital signal processing circuit 5 under control of the personal computer 4 outputs send data D1 as serial data to the IC card 2 and processes received data D2 coming as serial data from a reception circuit 6. In inputting and outputting the send data D1 and receive data D2, the digital signal processing circuit 5 requests the IC card 2 to respond for mutual authorization and other processes. Such processes also are used to output retrieved data from the IC card 2 to the personal computer 4 and to update the contents of the card as directed by the personal computer 4.
A transmission circuit 7 receives the send data D1 from the digital signal processing circuit 5 and modulates the data D1 according to a scheme in effect for wireless communication with the IC card 2. The circuit 7 thus generates for output a send signal S1 whose level varies depending on the send data D1. The method used here for data modulation is based illustratively on Manchester encoding. A Manchester code series is brought about by phase modulation, whereby the signal level is inverted across the middle of each bit cell, as shown in FIGS. 12A and 12B. That is, the signal level is switched between a logical 0 and a logical 1.
A wireless interface 8 generates a modulated signal by amplitude-modulating a suitable carrier signal based on the send signal S1. The wireless interface 8 drives the antenna using the modulated signal thus generated so as to transmit the send signal S1 to the IC card 2. Illustratively, the reader/writer 3 supplies the antenna with a carrier signal of a constant amplitude and switches the terminal impedance of the antenna as per the send signal S1, thereby generating the amplitude-modulated signal to drive the antenna.
As the antenna is being fed with the carrier signal of constant amplitude, the wireless interface 8 subjects a high-frequency signal induced on the antenna to amplitude detection, so as to generate a detected signal. The wireless interface 8 further puts the detected signal into binary format to generate a receive signal S2.
The reception circuit 6 regenerates a clock signal from the receive signal S2 and latches the receive signal S2 successively in reference to the clock signal, thereby decoding the receive data D2 sent from the IC card 2. In this manner, the reader/writer 3 exchanges data wirelessly with the IC card 2.
In the IC card 2, a wireless interface 10 detects a high-frequency signal induced on an antenna to generate a detected signal. The interface 10 further subjects the detected signal to binarization to output a receive signal S3. As the IC card 2 physically approaches the reader/writer 3 and as the amplitude of the high-frequency signal induced on the antenna becomes greater than a predetermined value, the receive signal S3 has its signal level inverted to correctly reflect the logic level of the send data D1.
Furthermore, the wireless interface 10 generates a modulated signal by amplitude-modulating a suitable carrier signal according to a send signal S4 output from a transmission circuit 11. The circuit 10 drives the antenna using the modulated signal, thereby transmitting the send signal to the reader/writer 3. The IC card 2 modulates in amplitude the high-frequency signal induced on the antenna by illustratively switching the terminal impedance of the antenna as per the send signal S4, whereby the send signal S4 is transmitted to the reader/writer 3.
A reception circuit 12 regenerates a clock signal from the receive signal S3 and latches the receive signal S3 successively in reference to the clock signal, thereby decoding the receive data D3 corresponding to the send data D1 from the reader/writer 3.
A digital signal processing circuit 13 outputs send data D4 to the transmission circuit 11 in response to the receive data D3, thus responding to a call from the reader/writer 3 and carrying out mutual authorization with the reader/writer 3. During the processing, the digital signal processing circuit 13 reads data from an internal memory for output to the reader/writer 3 and updates the memory contents as directed by the reader/writer 3.
The transmission circuit 11 modulates the send data D4 from the digital signal processing circuit 13, so as to generate the send signal S4 whose signal level varies depending on the send data D4. The method used here for data modulation is the same as that of the reader/writer 3, i.e., the method based illustratively on Manchester encoding. In this manner, the IC card system 1 permits wireless exchanges of appropriate data between the reader/writer 3 and the IC card 2.
FIG. 13 is a block diagram of the reception circuit 6 (and 12). The IC card 2 and the reader/writer 3 each generate a detected signal S6 (FIG. 14B) by detecting a high-frequency signal S5 induced on the respective antenna as an amplitude-modulated signal (FIG. 14A). The detected signal S6 is put into binary format with respect to a predetermined signal level, whereby the receive signal S2 (S3) is generated (FIG. 14C).
The reception circuits 6 and 12 each regenerate a clock signal by having the receive signals S2 and S3 input to a clock generation circuit 15. The clock generation circuit 15 located in an oscillation circuit 16 generates a clock signal CK (FIG. 14E) having substantially the same frequency as that for the receive signals S2 and S3. A phase synchronization circuit 17 compares the clock signal CK with the receive signals S2 and S3 in terms of phase. Given results of the phase comparison, the clock generation circuit 15 provides phase control over the clock signal CK, thus constituting a feedback loop circuit for clock signal (CK) regeneration.
The clock generation circuit 15 further generates a latch pulse P1 (FIG. 14D) that rises upon elapse of one-fourth of one clock cycle following each trailing edge of the clock signal CK from the oscillation circuit 16. A supplementary circuit 18 made of a latching circuit latches the receive signals S2 and S3 successively in reference to the latch pulse P1, thereby decoding the receive data D2 and D3 for output (FIG. 14D).
In the IC card system 1 above, external noise and other disturbances can degrade the S/N ratio of the high-frequency signal induced on the antenna and can abruptly change the signal level of that high-frequency signal. In the IC card 2 and reader/writer 3, degradation of the high-frequency signal in the S/N ratio leads to a waveform distortion in the detected signal and, thus, lowers the quality of the latter. Degraded quality of the detected signal in turn deteriorates the duty ratio of the receive signals S2 and S3 and can trigger their jitters. This produces bit errors in the receive data D2 and D3 that have been obtained by processing of the receive signals S2 and S3.
Illustratively, if receive signals S2A and S3A with a duty ratio of 50% each are correctly input (as shown in FIG. 15A-1), a latch pulse signal P1 (FIG. 15B) may be used to latch the receive signals S2A and S3A successively, whereby receive signal D2A and D3A (FIG. 15C-1) are correctly decoded. On the other hand, if receive signals S2B and S3B have their duty ratio partially deteriorated when input (FIG. 15A-2), it is still possible to generate receive data D2B and D3B (FIG. 15C-2) by successively latching the receive signals S2B and S3B using the latch pulse signal P1. In this case, however, those portions of the receive signals S2B and S3B in which the duty ratio is deteriorated (indicated by arrow A in the figure) produce errors when decoded.
In the above type of wireless communication system, such bit errors in the receive data D2 and D3 caused by the deterioration of detected signal quality are dealt with by an error correction process that produces the send signals D1 and D3. However, as the degree of bit errors worsens, it becomes increasingly difficult for the error correction process to address the errors. Eventually, the data involved need to be retransmitted repeatedly to overcome the errors. This lowers the effective data transmission rate considerably.
Even such repeated data retransmissions may eventually become insufficient for correct data reception. In the end, it may become impossible to exchange data between the IC card 2 and the reader/writer 3.
In the IC card system 1, weak electromagnetic waves are used by the reader/writer 3 and IC card 2 to exchange data therebetween in close proximity. In that setup, a high-frequency signal induced on the antenna is picked up to obtain a detected signal that may degrade in quality. If bit errors caused by the deterioration of the detected signal in quality are reduced, the communicable distance between the reader/writer 3 and the IC card 2 may be increased correspondingly. That in turn makes system 1 more convenient to use. Illustratively, if the IC card system 1 is used in ticket inspection systems at railway stations, passengers carrying IC cards instead of tickers are allowed to pass rapidly through the wickets by the system, thus easing the congestion.